An example is shown in Fig. REF , where the number of neutrino flavours
\(N_{\rm eff}\)  and the sum of
neutrino masses
are taken as free parameters. We see that cosmology requires
relic neutrinos of at least three flavours and sets the limit on neutrino mass
\(m_\nu \lesssim 0.1\)  eV (neutrino oscillation data tell that
neutrinos with masses
above 0.1 MeV are degenerate in mass). The latest Planck analysis
gives [1]}
\(\sum _i m_{\nu _i} < 0.23~\mbox{eV} \; , \;\;\;\;\;\;N_{\rm eff} = 3.15 \pm 0.23 \; .\) 
